Ceramic substrate with integrated circuit bonded thereon

ABSTRACT

A ceramic substrate, suitable for use as a circuit substrate on which integrated circuit chips are bonded, which consists essentially of, based on the weight of the substrate, 0.5˜5.0% by weight of MgO, and 95.0˜99.5% by weight of the total of Al 2  O 3  and SiO 2 , the proportion of the Al 2  O 3  to the SiO 2  being in the range from 50:50 to 80:20 by weight.

This is a division of application Ser. No. 224,532, filed as PCTJP80/00078, Apr. 18, 1980, published as WO80/02343, Oct. 30, 1980,§102(e) filed Dec. 5, 1980, now abandoned which arose from anInternational Application designating the U.S., Ser. No. PCT/JP80/00078,filed Apr. 18, 1980.

DESCRIPTION BACKGROUND OF THE INVENTION

The present invention relates to a ceramic substrate for use as acircuit substrate on which chips for integrated circuits, e.g. LSI, areto be bonded, particularly a circuit substrate having a thermalexpansion coefficient which is similar to that of silicon, i.e. the chipmaterial of the integrated circuits.

Generally, a ceramic circuit substrate must not exhibit a low bulkdensity and many pores on the surface and within the body. This isbecause such pores have a tendency to increase imperfections of thepatterns formed directly on the surface of the circuit substrate whichpatterns interconnect the integrated circuits. Also, such pores maybecome filled with water, which reduces the reliability of theintegrated circuits bonded on the circuit substrate.

Recently, integrated circuits have been bonded directly on circuitsubstrates with high packaging density in order to improve thepropagation speed and the heat dissipation of devices. However, such adirect bonding method involves a difficulty in that, as the chip sizeincreases, the thermal stress generated during the bonding operationincreases and fracture the chip. This thermal stress is caused by thedifference between the thermal expansion of the chip and the substrateduring the bonding operation.

The thermal expansion coefficient of the alumina usually used as acircuit substrate is 7×10⁻⁶ /°C. from room temperature to 500° C., whichis twice as high as the 2.5˜3.5×10⁻⁶ /°C. coefficient in the sametemperature range for the silicon usually used as integrated circuitchips. Thus, alumina has a disadvantage in that the size of theintegrated circuits must remain small in order to avoid thermal stressdue to the termperature difference during the bonding operation.

Recently, mullite (3AL₂ O₃.2SiO₂), which has a thermal expansioncoefficient relatively near that of silicon, has been proposed as aceramic circuit substrate material, as disclosed in Japanese Patent LaidOpen No. 49-116599 specification. However, the thermal expansioncoefficient of mullite is 4.3×10⁻⁶ /°C. from room temperature to 500°C., which value is also higher than that of silicon. No other processesare known to produce the desired ceramic materials.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a ceramicsubstrate having a low thermal expansion coefficient.

Another object of the present invention is to provide a ceramicsubstrate having a low dielectric constant.

Still another object of the present invention is to provide a ceramicsubstrate having a high bulk density and no water absorption.

A further object of the present invention is to provide a ceramicsubstrate which can be produced by firing at a low temperature.

Other objects and advantages of the present invention will be apparentfrom the following description.

There is provided, according to the present invention, a ceramicsubstrate, consisting essentially of, based on the weight of thesubstrate, 0.5˜5.0% by weight of MgO and 95.0˜99.5% by weight of thetotal of Al₂ O₃ and SiO₂, the proportion of the Al₂ O₃ to the SiO₂ beingin the range of from 50:50 to 80:20 by weight.

The ceramic substrate according to the present invention exhibits astructure within the grain boundaries between the mullite crystals (3Al₂O₃.2SiO₂) are filled with cordierite (2MgO.2Al₂ O₃.5SiO₂). The ceramicsubstrate according to the present invention can be produced by usingcommercially available magnesia (MgO), alumina (Al₂ O₃) and Silica(SiO₂).

The thermal expansion coefficient of cordierite from room temperature to500° C. is 1˜2×10⁻⁶ /°C., which is lower than the 4.3×10⁻⁶ /°C. ofmullite, and the ceramic material of the present invention has a reducedthermal expansion coefficient, from room termperature to 500° C. of3.8˜3.9×10⁻⁶ /°C., which is very close to the 2.5˜3.5×10⁻⁶ /°C.coefficient of integrated circuit silicon chips.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of the Al₂ O₃ --SiO₂ --MgO system;

FIG. 2 is a graph showing the relationship between the thermal expansionand the temperature of the ceramic substrate materials of the presentinvention, as well as other materials;

FIG. 3 is a graph showing the relationship between the thermal expansioncoefficient and the weight proportion of Al₂ O₃ to SiO₂ of ceramicmaterials of MgO-Al₂ O₃ --SiO₂ having different contents of MgO;

FIG. 4 is a graph showing the relationship between the thermal expansioncoefficient and the content of Mgo of ceramic materials of MgO--Al₂ O₃--SiO₂ ;

FIG. 5 is a graph showing the relationship between the dielectricconstant and the content of MgO of ceramic materials of MgO--Al₂ O₃--SiO₂ ;

FIG. 6 is a graph showing the relationship between the bulk density andthe content of MgO of ceramic materials of MgO--Al₂ O₃ --SiO₂ fired atdifferent temperatures;

FIG. 7 is a graph showing the relationship between the water absorptionand the content of MgO of ceramic materials of MgO--Al₂ O₃ --SiO₂ firedat different temperatures;

FIG. 8A is a structural model of a mullite substrate composition; and

FIG. 8B is a structural model of a mullite substrate composition havinggrain boundaries which are filled with cordierite according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to theaccompanying drawings and, also, the present invention will be comparedwith the prior art.

The phase diagram of the Al₂ O₃ --SiO₂ --MgO system (FIG. 1) illustratescrystalline phases formed by mixing and firing alumina (Al₂ O₃), silica(SiO₂) and magnesia (MgO). The marks on each side of the diagramrepresent the weight ratio of two of the three components.

The present inventors found that mainly mullite (3Al₂ O₃.2SiO₂) 1 isformed, as described hereinafter, by mixing 50 ˜80% by weight of aluminaand 20˜50% by weight of silica, and firing them. Further, the inventorsfound that the grain boundaries between mullite crystals (3Al₂ O₃.2SiO₂)can be filled with cordierite (2MgO.2Al₂ O₃.5SiO₂) 2 by mixing 0.5˜5.0%by weight of MgO and 95.0˜99.5% by weight of the total of Al₂ O₃ andSiO₂ , the proportion of the Al₂ O₃ to the SiO₂ being in the range offrom 50:50 to 80:20 by weight, and firing the mixture. This weightproportion of Al₂ O₃ to Sio₂ according to the present invention issuitable for forming mainly mullite crystals. If the weight proportionof Al₂ O₃ to SiO₂ is outside the above mentioned weight proportion, thethermal expansion coefficient of the fired ceramics increases so muchthat it is not suitable for use as a circuit substrate. This is probablydue to the fact that alumina or silica remains in an unreacted statewithout forming mullite crystals.

The weight proportion of Al₂ O₃ to SiO₂ which is suitable for theformation of cordierite corresponds to the above-mentioned weightproportion of Al₂ O₃ to SiO₂ suitable for forming mainly mullite. If thecontent of MgO is less than the lower limit of the above-mentionedrange, almost no cordierite is formed, and the thermal expansioncoefficient and the bulk density remain almost the same as in the casewhere no MgO is included. On the other hand, if the content of MgO ismore than the upper limit of the above-mentioned range, spinel crystals(MgO.Al₂ O₃) 3 are formed, which increase the thermal expansioncoefficient of the ceramic material so much that it is not suitable foruse as a circuit substrate.

FIG. 2 is a graph showing the relationship between the thermal expansionand the temperature of materials for use as circuit substrates or chipsof integrated circuits. In FIG. 2, line A corresponds to alumina (Al₂O₃), and line B corresponds to mullite (3Al₂ O₃.2SiO₂). Line Ccorresponds to a ceramic material of the present invention, whichincludes 0.5 or 5.0% by weight of MgO and 95.0 or 99.5% by weight of Al₂O₃ and SiO₂, the weight proportion of the Al₂ O₃ to SiO₂ being71.8:28.2, and line D also corresponds to a ceramic material of thepresent invention, which includes 1.0% by weight of MgO, and 99.0% byweight of Al₂ O₃ and SiO₂, the weight proportion of the Al₂ O₃ to theSiO₂ being the same as with line C. Finally, line E corresponds tosilicon, which is used as integrated circuit chips to be bonded on thecircuit substrate, and line F corresponds to cordierite (2MgO.2Al₂O₃.5SiO₂).

Referring to FIG. 2, alumina A, which has conventionally been used inthe prior art as circuit substrate material, exhibits a higher thermalexpansion than silicon E and, consequently, a higher thermal stress.Contrary to this, the thermal expansion of the ceramic materials of thepresent invention C and D is lower than that of alumina A and mullite Band close to silicon E of the integrated circuit material, because thegrain boundaries between mullite crystals B are filled with cordieriteF, which has the lowest thermal expansion.

FIG. 3 is a graph showing the relationship between the thermal expansioncoefficient and the weight proportion of Al₂ O₃ to SiO₂ of ceramicmaterials of MgO--Al₂ O₃ --SiO₂ having different contents of MgO. InFIG. 3, line C corresponds to 0.5 or 5.0% by weight of MgO, and line Dcorresponds to 1.0% by weight of MgO. The weight proportions of Al₂ O₃to SiO₂ of both lines C and D are in the range of from 80:20 to 47:53.As can be seen from these curves, where the weight proportion of Al₂ O₃to SiO₂ is in the range of from 50:50 to 80:20 the thermal expansioncoefficient is low. This is because the thermal expansion coefficientincreases outside this range of Al₂ O₃ to SiO₂, when a considerableamount of alumina and silica are included in the unreacted state.

FIG. 4 is a graph showing the relationship between the thermal expansioncoefficient and the content of MgO of ceramic materials of MgO--Al₂ O₃--SiO₂. As shown in FIG. 4, the thermal expansion coefficient of theceramic material from room temperature to 500° C. is reduced when theceramic contains 0.5˜5.0% by weight of MgO and 95.0˜99.5% by weight ofthe total of Al₂ O₃ and SiO₂, and the weight proportion of the Al₂ O₃ tothe SiO₂ is 71.8:28.2. If the content of MgO is less than 0.5% byweight, cordierite is formed only in a small amount, and the thermalexpansion coefficient remains similar to mullite itself. If the contentof MgO is more than 5.0% by weight, spinel (MgO.Al₂ O₃) 3 is formed.Furthermore, if the content of MgO is over 10.0% by weight, the thermalexpansion coefficient increases to 4.8×10⁻⁶ /°C., which is larger thanthat of mullite itself.

The ceramic substrate according to the present invention exhibits athermal expansion coefficient from room temperature to 500° C. of3.8˜3.9×10⁻⁶ /°C., which is very close to the 2.5˜3.5×10⁻⁶ /°C. of thesilicon integrated circuit material.

FIG. 5 is a graph showing the relationship between the dielectricconstant (1 MHz, 20° C.) and the content of MgO of ceramic materials ofMgO--Al₂ O₃ --SiO₂. As can be seen from FIG. 5, the dielectric constantis 6.5˜6.6 when the content of MgO is in the range of 0.5˜5.0% by weightand the rest consists essentially of Al₂ O₃ and SiO₂, the weightproportion of which is 71.8:28.2. If the content of MgO is either moreor less than the above mentioned range, the dielectric constant of theceramic material increases. Thus, the ceramic material having thecontent of MgO according to the present invention has the advantage oflowering the dielectric constant from that of mullite, which improvesthe transmission line characteristics of circuits formed on the ceramicsubstrate.

FIG. 6 is a graph showing the relationship between the bulk density andthe content of MgO of ceramic materials of MgO--Al₂ O₃ --SiO₂ fired atdifferent temperatures. The content of MgO is expressed in % by weightand the rest consists esssentially of Al₂ O₃ and SiO₂, the weightproportion of which is 71.8:28.2. Curves a, b, c and d correspond tofiring temperatures of 1550°, 1500°, 1450° and 1400° C., respectively.The conditions under which the ceramic substrate was prepared and firedwill be described in the Examples below. As can be seen from the curves,the bulk density is increased when the ceramic materials include0.5˜10.0% by weight of MgO at a firing temperature higher than 1500° C.

FIG. 7 is a graph showing the relationship between the water absorptionand the content of MgO of ceramic materials of MgO--Al₂ O₃ --SiO₂ firedat different temperatures. The content of MgO is expressed in % byweight and the rest consists essentially of Al₂ O₃ and SiO₂, the weightproportion of which is 71.8:28.2. Curves a, b, c and d correspond tofiring temperatures of 1550°, 1500°, 1450° and 1400° C., respectively.As can be seen from the curves, the water absorption of the ceramic isconsiderably reduced when MgO is included in the range of 0.5˜20.0% byweight at a firing temperature higher than 1500° C. Further, the firingtemperature of ceramic material of MgO--Al₂ O₃ --SiO₂ having a high bulkdensity with no water absorption is relatively low, compared with thecase where mullite is obtained by firing at 1650° C. Consequently, theresulting ceramic material can be used as a circuit substrate.

Generally, the bulk density of a ceramic substrate corresponds inverselyto the porosity and its water absorption corresponds to the porosity.Therefore, the ceramic substrate according to the present inventionexhibits a low porosity.

FIG. 8A is a structural model of a mullite substrate composition andFIG. 8B is a structural model of the ceramic substrate composition ofthe present invention. The grain boundaries between the mullite crystals1 are filled with cordierite 2, as shown in FIG. 8B. Cordierite isformed in a liquid phase during the firing and is thereafter solidified.Although the ceramics of MgO--Al₂ O₃ --SiO₂ having a content of MgO evenhigher than 5.0% by weight, which have been fired at a temperature of1500°˜1550° C., exhibit a sufficiently low porosity to be used as asubstrate, the thermal expansion coefficient increases, due to theformation of spinel crystals, to such an extent that it is not suitablefor use as a circuit substrate.

In summary, the ceramic substrate according to the present invention,which includes appropriate amount of MgO, Al₂ O₃ and SiO₂ so as to formcordierite which fills in the grain boundaries between the mullitecrystals, exhibits the following advantages.

First, the thermal expansion coefficient of the ceramic substrate isvery near that of the integrated circuit silicon chips bonded on thesubstrate. Consequently, the thermal stress caused by the difference inthe thermal expansion is small and, therefore, the size of chips whichcan be bonded safely can be large.

Second, the high bulk density with no water absorption of the ceramicsubstrate makes it possible to reduce imperfections of the patternsformed directly thereon which interconnect the integrated circuits, andthe low dielectric constant of the ceramic substrate can improve thetransmission line characteristics of the interconnected circuits formedthereon over those formed on the prior alumina or mullite substrate.

Third, the ceramic substrate according to the present invention can beproduced by using commercially available raw materials and firing themat a relatively low temperature.

EXAMPLE 1

Alumina (Union Carbide Co.), silica (Wako Junyaku Co.) and magnesia(Wako Junyaku Co.) were used as raw materials for producing a ceramicsubstrate. 71.8% by weight of alumina powder and 28.2% by weight ofsilica powder were mixed in a polyethylene pot using alumina balls forfour hours. The mixed powder was fired in air, at 1300° C., for onehour, so as to form mullite crystals. The crystals were ground usingalumina balls in a polyethylene pot for 24 hours. One part by weight ofmagnesia powder was added to 99 parts by weight of the ground crystalpowder. Then, 10 parts by weight of butyral as a binder, 14 parts byweight of di-butyl phthalate as a plasticizer, 1.4 parts by weight ofOP-85R (Nippon Yushi Co.) as a deflocculant, as well as 38 parts byweight of methyl ethyl ketone, 27 parts by weight of methyl alcohol and8.5 parts by weight of butyl alcohol as solvents, were added to 100parts by weight of the powder in a polyethylene pot and milled usingalumina balls for 140 hours to form a slurry.

A green sheet was formed from the slurry using the Doctor blade castingmethod, prefired in air, at 1300° C., for one hour, and fired in air, at1550° C., for two hours. Cordierite was formed in the grain boundariesbetween the mullite crystals.

The fired ceramic substrate exhibited a thermal expansion coefficient of3.8×10⁻⁶ /°C. from room temperature to 500° C., a dielectric constant of6.5 (1 MHZ, 20° C.), a bulk density of 3.13 g/cm³ and a water absorptionof 0.00%, as shown in FIGS. 4, 5, 6 and 7, separately.

Au-paste 9791 (E. I. du Pont de Nemours & Co.) was printed by a screenprocess on an undercoat of Au-Pt paste 8895 (E. I. du Pont de Nemours &Co.) on the surface of the fired ceramic substrate (100×100 mm), and thesubstrate having the pastes thereon was fired in air, at 950° C., for 10minutes so as to form metallized layers on the substrate.

An NiCr-Au film was formed by a vaccuum evaporation process on thebottom surface of a silicon wafer with a 76 mm diameter, which isusually used for the production of integrated circuits. This siliconwafer was put on the metallized layer while Au-Sn alloy having a meltingpoint of 280° C. was inserted therebetween and the substrate was heatedup to 300° C. The silicon wafer was successfully bonded to the ceramicsubstrate according to the present invention.

EXAMPLE 2

Ceramic substrates were prepared in a manner similar to that mentionedin Example 1, except that the weight proportion of Al₂ O₃ to SiO₂ wasvaried in the range of from 47:53 to 80:20, and the contents of Mgo were0.5, 1.0 and 5.0% by weight. The relationship between the thermalexpansion coefficient and the weight proportion of Al₂ O₃ to SiO₂ isshown by curves in FIG. 3.

EXAMPLE 3

Ceramic substrates were prepared in a manner similar to that mentionedin Example 1, except that the content of Mgo was varied in the range offrom 0.05 to 20% by weight. The relationship between the thermalexpansion coefficient and the content of MgO is shown in FIG. 4 and therelationship between the dielectric constant and the content of MgO isshown in FIG. 5.

EXAMPLE 4

Ceramic substrates were prepared in a manner similar to that mentionedin Example 3, except that the firing was carried out at 1400°, 1450°,1500° and 1550° C. The relationship between the bulk density, thecontent of MgO and the firing temperature is shown in FIG. 6 and therelationship between the water absorption, the content of MgO and thefiring temperature is shown in FIG. 7.

What is claimed is:
 1. An electronic device comprising as maincomponents an integrated circuit silicon chip bonded on a ceramicsubstrate, which consists essentially of, based on the weight of thesubstrate, 0.5˜5.0% by weight of MgO and 95.0˜99.5% by weight of thetotal of Al₂ O₃ and SiO₂, the proportion of the Al₂ O₃ to the SiO₂ beingin the range from 50:50 to 80:20 by weight said ceramic substrateexhibiting a structure in which the grain boundaries between the mullitecrystals (3Al₂ O₃.2SiO₂) are filled with cordierite (2MgO.2Al₂O₃.5SiO₂), said substrate having a thermal expansion coefficient of3.8˜3.9×10⁻⁶ /°C. from room temperature to 500° C., which coefficient isclose to that of the silicon chip, viz. 2.5˜3.5×10⁻⁶ /°C. from roomtemperature to 500° C.